The invention relates to a circuit arrangement for energizing a lamp, comprising
input terminals which are to be connected to a supply voltage source,
a first circuit part I coupled to the input terminals for generating a current through the lamp from a supply voltage supplied by the supply voltage source,
a second circuit part II coupled to the first circuit part I for setting the power consumed by the lamp to a desired value.
Such a circuit arrangement is disclosed in EP 0430357. In the known circuit arrangement, the power consumed by the lamp is regulated by measuring the actual lamp power and comparing this with the desired value. The result of this comparison is used to influence the operating condition of the first circuit part I in such a manner that the actual power consumed by the lamp is continuously substantially equal to the desired value of the lamp power. Viewed over a small number of operating hours of the lamp, such a substantially constant value of the power consumed by the lamp also means that the luminous flux of the lamp is substantially constant. However, viewed over a comparatively large number of operating hours, the luminous flux of the lamp decreases, at a constant lamp power, as a result of aging of the lamp. Apart from said decrease of the luminous flux as a result of aging of the lamp, the overall quantity of light may also decrease as a result of, for example, fouling of the luminaire accommodating the lamp. Often, such a reduction of the total quantity of light is taken into account in that the desired value of the power consumed by the lamp is set to a comparatively high value. As a result, although the quantity of light emitted by the lamp has decreased after a comparatively large number of burning hours, it still meets the requirements imposed, for example, for safety reasons. A drawback of dealing with the problem in such a way resides in that, during this comparatively large number of burning hours, the power consumption of the lamp is higher than would be necessary to generate a quantity light that satisfies the prevailing safety requirements. As a result, the operation of the lamp using the known circuit arrangement is comparatively inefficient.
It is an object of the invention to provide a circuit arrangement which enables a lamp to generate a quantity of light, throughout its service life, which satisfies the requirements to be imposed, while the lamp operates comparatively efficiently throughout the service life.
To achieve this, a circuit arrangement of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the second circuit part comprises a timer for measuring the total number of burning hours of the lamp, and a third circuit part III, coupled to said timer, for setting the power consumption of the lamp at the desired value in dependence upon the number of burning hours of the lamp.
In the course of the service life of the lamp, the third circuit part III increases the desired value of the power consumed by the lamp. As a result, the reduction of the amount of light generated due to aging and fouling is at least partly compensated for. If a circuit arrangement in accordance with the invention is used, the desired value of the power consumed by the lamp can be set, immediately after the lamp has come into operation (i.e. after zero burning hours), at a value such that the amount of light supplied by the lamp is sufficient, yet not much larger, to satisfy, for example, safety requirements. Before the amount of light supplied by the lamp decreases as a result of aging and fouling to a level below that required by safety requirements, the desired value of the power consumed by the lamp is increased by the third circuit part III. It is thus achieved that, throughout its service life, the lamp consumes approximately as much power as it needs to produce a desired/required amount of light. As a result, the lamp operates very efficiently. The increase of the desired value of the lamp power can take place continuously. Alternatively, such an increase can also take place in steps after a predetermined number of burning hours. In the latter case, said third circuit part advantageously comprises a memory for establishing a table that determines the relation between the overall number of burning hours and the desired value of the power consumed by the lamp. Said third circuit part also advantageously comprises a microprocessor enabling such a table to be read and the content of the timer to be monitored.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.